1. Field of the Invention
The present invention relates to a method of and an apparatus for manufacturing a molded product of foamed resin by injecting a molten resin into a mold and causing the molten resin to foam in the mold.
2. Description of the Related Art
Molded products of synthetic resin that are manufactured by injection molding or the like are advantageous in that they can be shaped considerably freely to various configurations. Therefore, molded products of synthetic resin are used in a wide range of fields and are available in a very large variety of product shapes. Many molded products of synthetic resin have integral convex portions such as bosses, ribs, etc. When such molded products are produced, those convex portions tend to suffer defects. Specifically, convex portions that are formed when recesses in mold cavities are filled with a resin are liable to have defects such as sink marks or the like that are a phenomenon in which a partial dent is created due to shrinkage upon cooling of the resin. Any defects that occur on the outer surface, which provides an outward appearance, of molded products are highly likely to spoil the products.
One process of minimizing such sink marks is a foaming process in which a resin is foamed to suppress shrinkage of the resin under the foaming pressure. According to the foaming process, the dimensional accuracy of the foamed product increases as the shrinkage of the resin is suppressed, and the foamed product is made lighter in weight and less costly because the amount of the resin used is reduced due to air bubbles included in the product. According to the foaming processes disclosed in Japanese patent No. 2625576 (first prior art) and Japanese laid-open patent publication No. 277298/97 (second prior art), no chemical foaming agent is used, but a supercritical fluid is introduced into a resin and dissolved in the resin, after which the resin is injected into a mold. According to these disclosed processes, fine cells are formed to produce a foamed resin product having a smaller cell size and a high cell density.
According to an injection molding process, when a molten resin is injected into a mold, since the mold has generally been cooled in advance, the injected resin is quickly cooled and solidified in the mold. For this reason, the shape of the mold may not be sufficiently transferred to the resin, and the resulting product may have a poor appearance owing to defects such as a weld mark on the surface of the molded product. As a solution to the problem of such appearance defects, there have been proposed processes of keeping a mold at a high temperature. For example, Japanese laid-open patent publication No.18229/2001 (third prior art) discloses an arrangement for and a method of introducing a heating medium into a flow channel defined in a cavity block disposed in a mold base of a mold when the mold is filled with a molten resin, and thereafter introducing a cooling medium into the flow channel for solidifying the filled resin. Japanese laid-open patent publication No. 348041/99 (fourth prior art) reveals an arrangement for and a method of alternately introducing a heating medium and a cooling medium into another flow channel disposed in a position near the surface of the mold in addition to a flow channel which is supplied with a cooling medium.
According to the foaming process, described above, that is used to prevent sink marks, if the temperature of the surface of the mold is low when the molten resin is injected into the mold, then appearance defects such as transfer failures D and weld mark W as shown in FIG. 1 of the accompanying drawings are likely to occur on the molded product. These defects according to the foaming process may be eliminated by increasing the temperature of the surface of the mold when the molten resin is injected into the mold, as with the third prior art and the fourth prior art. However, though the above approach is effective to eliminate appearance defects such as transfer failures D and weld mark W, a phenomenon of cell breaking which makes the surface poor is liable to occur, as indicated by broken cell marks V in FIG. 2 of the accompanying drawings. Specifically, when the temperature of the mold increases, the resin that is held in contact with the cavity surface of the mold increases its temperature and is foamed, and the membrane of produced cells is ruptured, producing broken cells. The broken cells leave broken cell marks V on the surface of the molded product, making its appearance very poor.